Pittsburgh Steelers strong safety Troy Polamalu (43) sits on the sidelines during the fourth quarter of an NFL football game against the Cleveland Browns in Pittsburgh, Sunday, Dec. 30, 2012. The Steelers won 24-10. (AP Photo/Gene J. Puskar)

The Pittsburgh Steelers are reloading with young players on their vaunted defense, looking to inject energy into the NFL's oldest starting unit last season.

A transition has begun with the additions of touted draft picks Jarvis Jones, Shamarko Thomas and Terry Hawthorne, but the quickest path to a turnaround lies with a number of veterans.

Count safeties Troy Polamalu and Ryan Clark among the elder statesmen out to prove themselves all over again.

Polamalu has had a Hall of Fame career, but comes off a season in which he missed nine games with a calf injury. The season before, he accounted for three turnovers in 16 games after forcing eight in 14 games the season before.

At his best, Polamalu is a hard-hitting, ball-hawking safety who makes every player better. Even at the age of 32, Polamalu has the ability to roam and make plays anywhere on the field like few other players in the NFL.

Call Polamalu and Clark the "old guard" in Pittsburgh, two players who have combined for 18 years of service time for the Steelers, but are still playing for their futures.

The Steelers have tough decisions to make next offseason and could realistically cut ties with both starting safeties. Clark and Polamalu are out to make that decision as difficult as possible by leading a defensive resurgence.

We'll take a look at their roles in the defense and the players who fall behind them on the depth chart with the latest in our training camp preview series.

PROJECTED STARTERS

SS Troy Polamalu: Polamalu, 32, was limited to just seven games last season because of a calf injury. He was the NFL's defensive player of the year in 2010 after recording 64 tackles and seven interceptions in 14 games, proving his unique value as an equally adept defender against the run or the pass. A future Hall of Famer, Polamalu's role as a versatile playmaker who can attack from anywhere eon the field is a key to unlocking Dick LeBeau's pressure defense.

FS Ryan Clark: Clark, 33, has expressed a desire to retire in Pittsburgh but is signed only through the end of the season. He's been a consistent presence in the Steelers' defensive backfield, earning a Pro Bowl nod in 2011 when he recorded the first of back-to-back 100-tackle seasons. Clark has made 75 starts at free safety over the last five seasons.

KEY RESERVES

FS Robert Golden: Golden signed with the Steelers as an undrafted free agent last season. He went on to play in three games at safety, where he recorded five tackles. Capable of playing either free or strong safety, Golden could be the next in line if either Clark or Polamalu suffers an injury.

SS Da'Mon Cromartie-Smith: Cromartie-Smith has spent the bulk of his time in Pittsburgh on the practice squad, but gets a shot for more with the departures of Will Allen and Ryan Mundy. He's played in six games with the Steelers, including two appearances on special teams last season. Cromartie-Smith originally signed with the Steelers as an undrafted free agent in 2010.

NEW FACES TO WATCH

SS Shamarko Thomas: Thomas is cut from the same cloth as Polamalu as a compact, versatile safety who can make plays from anywhere on the field. A fourth-round pick, Thomas was a standout at the NFL combine, where he led all defensive backs in the 40-yard dash (1st, 4.42 seconds), bench press (1st, 28 reps) and vertical jump (T-1st, 40.5 inches). He's considered by many as the heir apparent to Polamalu and a potential impact player.

FS Ross Ventrone: A Pittsburgh native, Ventrone began his NFL career with the New England Patriots, who proceeded to sign and release him more than 21 times last season, according to this VUHoops.com post. He signed a future/reserve contract with the Steelers in January.

KEY LOSSES

Ryan Mundy: Mundy was a four-year contributor for the Steelers before signing with the New York Giants in March. He made a career-high three starts last season and led all defensive backs with 307 snaps played on special teams.

Will Allen: Allen, 30, came to Pittsburgh as a free agent in 2010, where he became one of the team's top reserves last season. Allen made seven starts and recorded 31 tackles while also playing a majority of snaps on special teams.